CHAP. Il] THE MAMMALIA 17 



been built up systems familiar to us as the skeletal system, 

 digestive system, and so on. It is indeed an important con- 

 sideration in Vertebrate Morphology, that specialization of 

 primitively similar cells occurs, resulting in the production 

 of tissues and systems. From the general consideration of the 

 vertebrate body it is however necessary to pass to that of the 

 several tissues and systems. 



In the study of the morphology of the various systems we 

 find that each is constructed on a certain plan, so that each may 

 be referred to a fundamental, primitive, or type-form. Thus the 

 primitive forms of the central nervous system, or of the skeletal 

 or arterial systems, are common expressions. Each system of a 

 given animal reveals a more or less altered representation of 

 that primitive form. The point before us consists in the fact 

 that in such a given instance (and this is especially true of 

 Mammals), though each system will be found to represent a 

 modified form of the primitive type, yet the amount of modifica- 

 tion undergone is different for each system. Thus an animal may 

 possess a nervous system of very primitive, i.e. slightly modified, 

 form together with an extremely modified integumentary system. 

 It follows therefore that in assigning a place in the classification 

 to a given animal, attention must be paid to the morphology of 

 each and every system, and a position must only be assigned to it 

 after a final summary of all the evidence has been made. Thus 

 an animal like the hedgehog presents us with the morphological 

 combination of a brain of simple conformation, together with 

 an integument of very modified character. Taking other mor- 

 phological points in the anatomy of the hedgehog, the general 

 balance of the summing up of evidence indicates that on the whole 

 this is a little modified, i.e. a comparatively simply-constructed 

 animal, which is the result one would have arrived at from the 

 study of the brain alone, though opposed to the indication that 

 would be afforded if the observation had been confined to the 

 integument. 



The disadvantage attendant on the employment of a single 

 criterion, such as the conformation of a single organ or system, 

 may be further illustrated by the example of the genitalia 

 in Man. Judged by the single characteristic of the morphology 



D. M. 2 



